The term “NOx” is used herein to refer to any of the nitrogen oxides NO, NO2, N2O, or mixtures of two or more thereof. Over the past ten years, increasingly more stringent heavy duty on-highway engine emission regulations have led to the development of engines in which NOx and diesel particulate emissions have been reduced by as much as 70% and 90%, respectively.
Proposed regulations for new heavy duty engines require additional NOx and diesel particulate emission reductions of over 70% from existing emission limits. These emission reductions represent a continuing challenge to engine design due to the NOx-diesel particulate emission and fuel economy tradeoffs associated with most emission reduction strategies.
Emission reductions are also being sought from the on and off-highway in-use fleets. Within the heavy duty engine population, it is becoming more apparent that older engines are contributing a disproportionate amount of emissions toward the overall mobile source emissions inventories. Many of these engines have useful lives of over 15 years. To meet the air quality objectives in many regional areas, reductions in NOx and diesel particulate emissions will need to be derived from the in-use, mobile source engine population. In some regional areas, the proposed or required emission reductions from the engines in use potentially represent an even greater challenge than the emission limits proposed for new engines.
It is recognized that the engine, lubricants, fuel, aftertreatment and the engine application must be integrated into a system to maximize the control of emissions.
Recent engine work focuses on improvements or incorporation of new technologies to the power cylinder, air delivery, fuel management, and electronic systems. These improvements typically satisfy the emission requirements of new engines. Some improvements can also be implemented during the engine rebuild process; however, internal engine modifications are not broadly and practically applicable to in-use fleets.
Some new engines require the use of diesel specific oxidation catalysts to meet diesel particulate matter limits. This technology can be readily retrofitted.
In the urban environment, health concerns regarding diesel particulate emissions are resulting in thousands of buses being retrofitted with oxidation catalyst technologies. Similarly, natural gas buses frequently employ oxidation catalysts to maintain low particulate, carbon monoxide and hydrocarbon emissions.
In the United States, the implementation of the urban bus rebuild/retrofit requirements increased interest in emission reduction technologies that reduce emissions from in-use heavy duty diesel vehicle fleets.
In the United Kingdom and Sweden, many oxidation catalysts are also being installed in retrofit programs.
The retrofitting of heavy duty diesel engines with oxidation catalyst technologies has been utilized for on and off-highway vehicles used in mining, materials handling and other industrial markets. Worker health and safety is the primary reason for these applications.
A rapid series of diesel fuel improvements has been introduced in most parts of the developed world to provide reductions in particulates and NOx from the vehicle fleets in current operation as well as to facilitate the introduction of aftertreatment devices. Reducing the sulfur content and the “heavy end” of the fuel have been the key changes. In the U.K., government tax incentives have initiated the widespread use of a new grade of diesel fuel termed ultra low sulfur diesel (ULSD), which has a maximum 50 parts per million (ppm) sulfur content and a 95% distillation temperature of less than 345° C. As well as achieving immediate reductions in particulates and NOx from the current vehicle fleet, the availability of ULSD was intended to encourage the adoption of the latest exhaust oxidation catalysts whose operation is sensitive to higher fuel sulfur levels.
Diesel fuel improvements typically involve the reduction of fuel sulfur via hydrotreating to levels as low as 10 ppm (Swedish Mk 1 fuel). Other fuel parameters such as aromatics and cetane have also been the subject of investigation. Specially manufactured fuels and the incorporation of special fuel components such as biodiesels, Fisher Tropsch blends, methanol, and ethanol, are also gaining attention.
While many of the foregoing suggestions are meritorious, the problem remains that further reductions in pollutants, especially NOx and diesel particulate emissions, are required. The inventive process provides a solution to this problem.